The search for new electrically active materials is driven from a wide range of potential applications.
Safe and powerful energy storage devices are becoming ever increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries can be provided by electrochemical capacitors, in particular, pseudocapacitors. Recent research has focused primarily on improving gravimetric performance of electrodes, but for portable electronics and automobiles, volume is at a premium.
In the search for new electrode materials, two-dimensional, 2D, solids are of particular interest due to their large areas of electrochemically-active surfaces. For example, the use of activated graphene electrodes versus conventional porous carbons can result in capacitances of 200-350 F/cm3 (compared to 60-100 F/cm3 for activated carbons). Yet graphene is limited to the chemistry of carbon, does not tap into metal redox reactions as in RuO2, and its conductivity is substantially decreased by redox-active functional groups.
The best volumetric capacitances of carbon-based electrodes are in the 300 F/cm3 range. Hydrated ruthenium oxide, RuO2, utilizes highly reversible redox reactions to reach capacitances in the 1000-1500 F/cm3 range combined with great cyclability, but only for thin films.
It is an object of the present invention to address at least some of these challenges, or to provide a useful alternative.
In other applications, conductors that are extremely thin can be transparent and the fabrication of transparent conductors (TCs), a critical element of touchscreen electronics and solar cells, is a billion dollar per year industry. Transparent conducting films (TCFs) are optically transparent and electrically conductive in thin layers. They are an important component of a number of electronic devices including liquid-crystal displays, light-emitting diodes, touchscreens and photovoltaics.
Transparent conducting films can be used as windows through which light passes to access a photoactive material beneath (e.g., a photovoltaic, where carrier generation occurs), as an ohmic contact for carrier transport out of the photoactive material, and can also act as transparent carrier for surface mount devices used between laminated glass or light transmissive composites.
Indium-tin-oxide (ITO) is the most widely used transparent conductors but is limited by the high cost of both the raw materials, such as indium, and the fabrication technique. Fluorine doped tin oxide (FTO), and doped zinc oxide have also been used for such applications. More recently, films using materials such as silver nanowires or carbon nanotube networks or graphene have been used as an alternative to ITO. Such materials are particularly useful owing to their transparency to infrared light. These all represent ‘bottom-up’ nanomaterials, requiring expensive synthetic procedures to make the starting material. The highest conductivity reported for solution processed graphene is 200 S/cm and this material has not been shown to demonstrate transparency in any applications.
It is an object of the present invention to address at least some of these challenges, or to provide a useful alternative.